1. Field of the Invention
This invention relates generally to chemical mechanical planarization apparatuses, and more particularly to methods and apparatuses for improved edge performance using an air bearing with a raised topography to constrain airflow under a wafer.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform chemical mechanical planarization (CMP) operations. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material grows. Without planarization, fabrication of further metallization layers becomes substantially more difficult due to the variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then, metal CMP operations are performed to remove excess material.
A chemical mechanical planarization (CMP) system typically is utilized to polish a wafer as described above. A CMP system generally includes system components for handling and polishing the surface of a wafer. Such components can be, for example, a rotary polishing pad, an orbital polishing pad, or a linear belt polishing pad. The pad itself typically is made of a polyurethane material or polyurethane in conjunction with other materials such as, for example, a stainless steel belt. In operation, the belt pad is put in motion and a slurry material is applied and spread over the surface of the belt pad. Once the belt pad having slurry on it is moving at a desired rate, the wafer is lowered onto the surface of the belt pad. In this manner, wafer surface is substantially planarized. The wafer may then be cleaned in a wafer cleaning system.
FIG. 1A shows a linear polishing apparatus 10 typically utilized in a CMP system. The linear polishing apparatus 10 polishes away materials on a surface of a semiconductor wafer 16. The material being removed may be a substrate material of the wafer 16 or one or more layers formed on the wafer 16. Such a layer typically includes one or more of any type of material formed or present during a CMP process such as, for example, dielectric materials, silicon nitride, metals (e.g., aluminum and copper), metal alloys, semiconductor materials, etc. Generally, CMP may be utilized to polish the one or more of the layers on the wafer 16 to planarize a surface layer of the wafer 16.
The linear polishing apparatus 10 utilizes a polishing belt 12, which moves linearly with respect to the surface of the wafer 16. The belt 12 is a continuous belt. A motor typically drives the rollers so that the rotational motion of the rollers 20 causes the polishing belt 12 to be driven in a linear motion 22 with respect to the wafer 16.
A wafer carrier 18 holds the wafer 16, which is held in position by mechanical retaining ring and/or by vacuum. The wafer carrier positions the wafer atop the polishing belt 12 so that the surface of the wafer 16 comes in contact with a polishing surface of the polishing belt 12.
FIG. 1B shows a side view of the linear polishing apparatus 10. As discussed above in reference to FIG. 1A, the wafer carrier 18 holds the wafer 16 in position over the polishing belt 12 while applying pressure to the polishing belt. The polishing belt 12 is a continuous belt typically made up of a polymer material such as, for example, the IC 1000 made by Rodel, Inc. layered upon a supporting layer. The rollers 20 rotate, moving the polishing belt in the linear motion 22 with respect to the wafer 16. In one example, a fluid bearing platen 24 supports a section of the polishing belt under the region where the wafer 16 is applied. The platen 24 can then be used to apply fluid against the under surface of the supporting layer of the belt pad. The applied fluid thus forms a fluid bearing that creates a polishing pressure on the underside of the polishing belt 12 that is applied against the surface of the wafer 16.
The above described linear polishing apparatus 10 functions well for most CMP operations when used with a supported polishing belt 12, such as a stainless steel belt having a polymer material covering. However, more efficient polishing belts 12 are currently available that are not supported. Since supporting material, such as stainless steel, does not form part of an unsupported polishing belt 12, unsupported polishing belts 12 often are easier to ship, higher quality, and less expensive to construct. As a result, unsupported polishing belts 12 generally are desirable to use in linear polishing systems.
Unfortunately, current linear polishing apparatuses 10 often perform poorly when polishing copper layers using an unsupported polishing belt 12. For example, FIG. 1C is an illustration showing an edge of a wafer 16 having a copper layer. The exemplary wafer 16 edge includes a copper layer 50 disposed over a dielectric layer 52. As is well known in the art, a slight raised section 54 occurs on copper layers 50 near the edge of the wafer 16 because of the particular properties of copper. As a result, it is desirable to increase the removal rate of the polishing process near the edge of the wafer during planarization of copper layers 50 to planarize the raised section 54.
Prior art linear polishing apparatuses generally can achieve an increased removal rate along the edge of the wafer 16 using a supported polishing belt, as illustrated in FIG. 2A. FIG. 2A is a graph 200 showing a removal rate using a supported polishing belt as a function of the distance from the center to the edge of a wafer. When using a supported polishing belt, such as a stainless steel supported polishing belt, the removal rate at the edge of the wafer can be increased dramatically, as shown in graph 200. In particular, the removal rate can be increased at a wafer radius of about 90 mm, which is about the radius of the slight raised section, which occurs on copper layers near the edge of the wafer.
However, as discussed previously, more efficient polishing belts are currently available that are not supported. As a result, unsupported polishing belts generally are desirable to use in linear polishing systems. Unfortunately, as mentioned above, conventional linear polishing apparatuses often perform poorly when polishing copper layers using an unsupported polishing belt, as illustrated in FIG. 2B. FIG. 2B is a graph 250 showing removal rate using an unsupported polishing belt as a function of the distance from the center to the edge of a wafer. As shown in graph 250, when using an unsupported belt in a conventional linear polishing apparatus, the removal rate at a wafer radius of about 90 mm is still slow and does not increase significantly until about 95-97 mm, which is beyond the radius of the slight raised section in the copper layer 50. As a result, it is difficult to effectively polish a copper layer 50 using an unsupported belt in a conventional linear polishing apparatus.
In view of the foregoing, there is a need for an apparatus that allows effective polishing of copper layers using unsupported polishing belts.
Broadly speaking, the present invention fills these needs by providing an air bearing platen with a raised topography to constrain airflow under a wafer. The raised topography of the platen allows enhanced edge removal rate uniformity control when using an unsupported polishing belt. In one embodiment, a CMP apparatus for enhancing removal rate uniformity is disclosed. The CMP apparatus includes a polishing belt disposed below a carrier head that is capable of applying a wafer to the polishing belt. Also included is a platen disposed below the polishing belt. The platen includes a circular shim section disposed on the top surface of the platen. The circular shim section is higher than the top surface of the platen. When using this configuration, increasing pressure to the backside of the polishing belt decreases the edge removal rate of the wafer. Conversely, decreasing pressure to the backside of the polishing belt increases the edge removal rate of the wafer.
A raised topography platen for use in a CMP system is disclosed in an additional embodiment of the present invention. The raised topography platen has a top surface disposed below the polishing belt, and a circular shim section disposed on the top surface of the platen. As above, the circular shim section is higher than the top surface of the platen. In addition, the circular shim section is capable of contacting the backside of the polishing belt during a planarization operation. Optionally, fluid pressure apertures, which provide fluid pressure to the backside of the polishing belt, can be disposed in the top surface of the platen. In one aspect, fluid pressure can be provided only from fluid pressure apertures disposed within the circular shim section. In this case, a closed volume can be formed between the circular shim section, the top surface of the platen, and the backside of the polishing belt during a planarization operation. Also optionally, the circular shim section can be disposed on a circular shim mount, which can be capable of being removed from the platen. In this aspect, the circular shim section can be either incorporated into the circular shim mount, or capable of being removed from the circular shim mount.